1. Field of the Invention
The present invention relates to nerve stimulation for regional anesthesia administration and more particularly to a processor implemented nerve stimulation system which varies pulse charge over an attenuation range by varying pulse duration as a function of applied current or applied current as a function of pulse duration.
2. Antecedents of the Invention
The field of regional anesthesia relates to the practice of administering anesthesia to a specific body region during surgery, for the relief of postoperative pain, and for extended relief of trauma or chronic pain. Often, regional anesthesia has been found to be preferable to general anesthesia because of increased safety, the availability of postoperative pain control and decreased anesthetic costs.
Regional anesthesia delivery techniques strove to optimize administration of a local anesthetic in close proximity to a target or nerve plexus so as to establish a neural blockade. Successful administration of regional anesthesia was dependent upon the accurate placement of the anesthetic in relation to the target nerve or nerves.
Various techniques have been employed to assist in placement of an administration needle in close proximity to the target nerve, which was not externally visible. One of the traditional methods of needle placement involved eliciting paresthesia. Among the disadvantages of this technique was the lack of accurate patient responses amongst patients who were disoriented and/or sedated.
A prevalent technique employed the use of nerve stimulators electrically coupled to a nerve stimulator needle. Such method was premised upon the phenomenon that an electrical pulse is capable of stimulating a motor nerve fiber to contract an innervated muscle or cause paraesthesia, in the case of sensory nerve stimulation.
The nerve stimulator needle was placed within the tissue of the patient's body in the vicinity of the nerve to be blocked and then advanced until stimulation of the target nerve was achieved as determined by visually detecting muscle contractions or by eliciting a report that the patient felt the stimulus in response to the current flow through the stimulator needle.
The current supplied by the nerve stimulator was reduced as the nerve stimulator needle was further advanced, until stimulation of the target nerve was achieved using a reduced current level associated with a prescribed distance between the needle tip and the target nerve.
Conventional electrical nerve stimulation techniques have utilized a weak direct current electrical current supplied to a nerve stimulator needle by an oscillating (square-wave) current generator. The current was pulsed, typically at a frequency of 1 Hz or 2 Hz. Current amplitude was adjusted, for example, by a potentiometer with the frequency and pulse duration remaining constant. When appropriate motor contractions, which corresponded to the muscular innervation of the targeted nerve or plexus, occurred at the set frequency, the current was slowly decreased in amperage while the needle was advanced to search for the nerve. Motor contractions that occurred at a very low amperage (usually 0.2-0.5 mAmp) indicated close proximity or contact between the needle tip and the nerve. Thereafter, a portion of the anesthetic dose was administered through the needle to terminate the response to the nerve stimulation current. If the response was terminated by the initial administration, the needle was deemed to be properly positioned in proximity to the target nerve and the remaining dose of anesthetic was administered.
Pulse duration, i.e. pulse width, is the duration in milliseconds of the periodic pulse square wave used to stimulate the nerve or nerve plexus. Increasing the pulse duration or the current amplitude increased the total flow of electrons i.e. pulse charge, in a manner proportional to the area under the square waveform. Increasing pulse duration therefore resulted in increased ability to stimulate the nerve at a greater distance which was a function of the pulse width, if other parameters, including current amplitude, are maintained constant. Greater pulse width (e.g. 0.3 msec compared to 0.1 msec) resulted in higher sensitivity to stimulate the designated nerve or plexus at a greater distance. By contrast, lower pulse width (e.g. 0.1 msec or 0.05 msec) maximized specificity for ultimate final location of the nerve relative to the needle tip. This assured optimal positioning of the needle.
The majority of nerve stimulators used have a preset single pulse duration (e.g. 0.1 msec or 0.2 msec). Some recent nerve stimulators permitted pulse duration to be set at a selected pulse width, e.g. 0.1 msec, 0.3 msec, 0.5 msec, or 1.0 msec.
A nerve stimulator that generated alternating sequential electrical pulses of high and low pulse widths was disclosed in U.S. Pat. No. 5,853,373. The high duration pulses were said to stimulate the target nerve at one-second intervals after skin penetration at a distance where the low duration pulses did not generate observable stimulation. When the target nerve was approached, both high and low duration pulses were said to generate observable stimulation (motor contractions) at one-half second intervals (twice the frequency initially observed), without a reduction in current amplitude.